I have scoured the Internet looking for this "thank you" letter. Would you please provide a copy for me to read. The only letter that I could find was the following and it did not contain any thank you note:

You said that once already, and you’re wrong. The WT thanked the
guardian for bringing the NGO changes to its attention in the 2001
letter, changes that violate their beliefs, and they withdrew.

There were no "NGO changes". There was nothing new brought to the attention of Watchtower. The criteria for NGO membership were the same in 1991 as they were in 2002. The same factors that you yourself admit "violate their beliefs" were at play in 1991.